IB CHEM 1 - Lesson 1 Part 3: Changes of State

What does KMT say about matter?

Matter’s properties come from particle energy, arrangement, and motion (collisions, spacing, IMF strength)

How are particles arranged/moving in a solid?

Closely packed, ordered; particles vibrate in place only; fixed shape & volume; high density; lowest energy

What makes liquids flow?

Particles are close but disordered and slide past each other; fixed volume, takes container’s shape; medium density

Why are gases compressible?

Particles are far apart, move fast/randomly; lots of empty space → compressible, very low density

What causes gas pressure?

Particle collisions with container walls; more frequent/forceful collisions → higher P

Do state changes alter identity?

No—physical, reversible; substance identity stays the same, only particle spacing/energy changes

Solid→liquid? Liquid→solid?

Melting; Freezing

Two paths from liquid to gas?

Vaporisation includes boiling (throughout, at a specific T when Pvap = Patm) and evaporation (surface, below bp)

Gas→liquid is…?

Condensation

Solid→gas? Gas→solid?

Sublimation; Deposition

Which state changes absorb/release energy?

Endothermic: melting, vaporisation, sublimation (overcoming IMFs). Exothermic: freezing, condensation, deposition (particles come closer)

Where does the energy go?

Into overcoming intermolecular forces (raising potential energy), not just raising temperature

Key differences?

Boiling: at bp, throughout liquid, needs Pvap = Patm. Evaporation: below bp, surface only, rate depends on T, surface area, airflow, humidity

When does liquid boil?

When vapor pressure = external pressure; lower external P (high altitude) → lower bp

Why is temperature flat during melting/boiling?

Energy goes to phase change (IMFs), not temperature; latent heat region

Which states compress easily? Why?

Gases (large spacings). Solids/liquids are nearly incompressible (tight packing)

Typical density order?

Solid > Liquid > Gas (water is an exception near 0–4 °C)

What changes across S→L→G?

Spacing increases, order decreases, motion/energy increase

What do (s), (l), (g), (aq) mean?

Solid, liquid, gas, aqueous (dissolved in water). Often scored in thermochemistry contexts

“(l)” vs “(aq)”—when to use?

(l) for pure liquids (e.g., H₂O(l) at bp). (aq) for solutions (e.g., NaCl(aq))

How to match energy flow with particle-level change?

IMFs weaken on endothermic changes; IMFs strengthen on exothermic changes—align description with energy direction

Name one.

Dry ice (CO₂(s)) → CO₂(g); iodine can also sublime

What speeds evaporation?

Higher T, larger surface area, airflow, lower humidity

Why does heating a sealed gas raise pressure?

Particles move faster → more frequent/forceful collisions